Permanent Magnet Alternators: Voltage, current, torque and RPM

    It is important to know the voltage and rpm specifications of the product. But exactly how are they related?

    Simply said, voltage generated in an alternator is directly proportional to the speed of the generator or the prime mover; E = K x Flux x speed x turns

    That means that on double the rpm of the generator and the AC voltage is doubled. Any permanent magnet alternator has its designed RPM at a rated voltage and load condition.

    The two factors that go into consideration when planning a complete energy system when using permanent magnet generators are the following:

    • An increase in rpm will increase the voltage.
• An increase in load will lower the voltage.

    There can be two alternators of the same size but a change in the winding will allow one to be designed to run at a low range of RPM and the other designed to run at very high RPM. They could be rated for different power and operate at different V or I.

    Voltage of a permanent magnet alternator varies on the length of the stator wire and the RPM of the alternator. But how the current amount varies? What makes a permanent magnet alternator/generate current high and low?

    Normally the alternator will have the magnet in the rotor so there will be a varying flux in the pole pieces in the stator. 
The voltage produced will depend on the rate of change of flux which will be dependent on the rotor speed and the number of turns on the pole pieces. A lot of turns gives a high voltage but the wire will be thin so current capability will be low. A few turns means the wire can be thicker so low voltage but high current.
The power will be the same in each case.
To get more power, get a bigger machine.
Once you have the machine working, you can get different voltages and current by using a transformer.

    Can we use a small motor to drive the big alternator to generate more power since the voltage is related to rpm, so we can get free energy?

    The law of conservation of energy tells that it is not possible.

    Yes, it’s true, higher rpm, higher voltage. But there is a very important element: Torque!

    The formula of rated torque is 9550 x rated power ÷ rpm x power factor. The higher load, the higher torque.

    Think about when riding a bike, you can make it very fast when there is nobody on it, but it difficult when a person sits on it, and more difficult with two persons on it.

    So if you are trying to use a 5kw 300rpm motor to drive a 50kw 300rpm alternator, don’t forget the torque of the machine.